Brain Teaser #1
A) Question:
12 MeV at 180 cGy per day is prescribed to the 90% line. The cone size is a 15x15 and at this facility all cone factors are normalized at 100 SSD for a 15x15 cone. The cutout factor is .981, and this was measured by physics. Unfortunately this is a head and neck electron boost, and the shoulders are in the way to achieve an SSD of 100. So, 110 SSD must be used. The virtual source distance is 100. The d-max of this electron beam is 2.5 cm for a 15X15 cone. Assume that this will obey the inverse square law. This is what the therapist came up with for the calculation. It is your job to second check it.
Cone factor=1.000
Cutout= .981
Percent line= .900
Distance Factor=1.205
A) Answer:
NO, the calculation is wrong.B) Question:
12 MeV at 180 cGy per day is prescribed to the 90% line. The cone size is a 15x15 and at this facility all cone factors are normalized at 100 SSD for a 15x15 cone. The cutout factor is .981, and this was measured by physics. Unfortunately this is a head and neck electron boost, and the shoulders are in the way to achieve an SSD of 100. So, 110 SSD must be used. The virtual source distance is 100. The d-max of this electron beam is 2.5 cm for a 15X15 cone. Assume that this will obey the inverse square law.
Cone factor=1.000
Cutout= .981
Percent line= .900
Distance Factor=1.205
B) Answer:
The distance factor is wrong.C) Question:
12 MeV at 180 cGy per day is prescribed to the 90% line. The cone size is a 15x15 and at this facility all cone factors are normalized at 100 SSD for a 15x15 cone. The cutout factor is .981, and this was measured by physics. Unfortunately this is a head and neck electron boost, and the shoulders are in the way to achieve an SSD of 100. So, 110 SSD must be used. The virtual source distance is 100. The d-max of this electron beam is 2.5 cm for a 15X15 cone. Assume that this will obey the inverse square law.
C) Answer:
Distance factor should be .830Follow Up:
-------------------------------------------------------------------------= df
(new SSD+d-max)2
(100+2.5)2
-------------------------------------------------------------------------=.830
(110+2.5)2
This distance factor comes from your virtual source distance which was 100 plus d-max divided by your desired SSD which in this case is 110 plus d-max. You may be asking what a virtual source distance is. Remember with electrons there is no target. The virtual source distance is the distance from isocenter to where the target would be (a very simple explanation). Keep in mind that the VSD may change from facility to facility and may not be 100 (always ask what the VSD is). As well, some facilities may want physics to measure the distance factor. This is because with small electron field sizes and depending on what the distance is, the inverse square law may not always hold true. Always check with your facility to see what their standard is for getting the distance factor. This can all be found in Khan page 371-374 (Physics of Radiation Therapy, second edition).
D) Question:
12 MeV at 180 cGy per day is prescribed to the 90% line. The cone size is a 15x15 and at this facility all cone factors are normalized at 100 SSD for a 15x15 cone. The cutout factor is .981, and this was measured by physics. Unfortunately this is a head and neck electron boost, and the shoulders are in the way to achieve an SSD of 100. So, 110 SSD must be used. The virtual source distance is 100. The d-max of this electron beam is 2.5 cm for a 15X15 cone. Assume that this will obey the inverse square law.
D) Answer:
MU=246Follow Up:
This is the equation you should have used:
Cone factor=1.000
Cutout= .981
Percent line= .900
Distance Factor=.830
E) Question:
The first day of treatment you gave 169 monitor units instead of the 246 monitor units you were supposed to give. What was the actual dose you delivered that day to the tumor instead of the 180 cGy you were supposed to deliver?E) Answer:
123.7 cGy to the tumor for that day.Follow Up:
This is just a simple proportion. You know the tumor dose was suppose to receive 180 cGy for 246 MU. You delivered 169 MU, so you can set up the following:--------- = ---------
246 MU 180 cGy
F) Question:
If only 123.7 cGy was given, then what was your percent error?F) Answer:
31.3% error.Follow Up:
--------------------------------------------- X100
accepted
180 cGy -123.7 cGy
--------------------------------------------- X100
180 cGy
In this case, your accepted is 180 cGy (that is what your doctor wants delivered to the tumor) and your actual is 123.7 cGy because you actually delivered that dose.
G) Question:
For this one day was the patient overdosed or underdosed?G) Answer:
The patient was underdosed.Follow Up:
The patient was underdosed, this is because you were supposed to give 180 cGy, but you really gave only 123.7 cGy.THE END
Author: Adam Buell
Editors: Mary Hare and Janelle Plavchan
Date Created: Febuary 2005
